In general, an insulating substrate, which is applied to a package for a semiconductor device etc. is made of alumina (Al.sub.2 O.sub.3). A lead frame is generally made of iron-nickel alloy such as Kovar (trade name for an Fe-29%Ni-17%Co alloy) or 42 alloy (Fe-42%Ni). The lead frame is brazed to a metal layer part of an insulating substrate, which is provided with a circuit. The brazing is performed with silver solder or the like for application to a package for a semiconductor device.
However, although alumina is excellent as an electric insulator and for mechanical strength, its heat dissipation property is inferior due to its small thermal conductivity of 30 W/mK. Thus, it is improper to mount a field-effect transistor (FET) of high calorific power, for example, on an alumina substrate. For mounting a semiconductor element of high calorific power, another type of insulating substrate is made of beryllia (BeO) having a high thermal conductivity of 250 W/mK. However, beryllia is toxic and hence it is troublesome to satisfy safety requirements in connection with such an insulating substrate.
In recent years, nontoxic aluminum nitride (AlN) has generated great interest as a material for making such insulating substrates for carrying a semiconductor element of high calorific power, because of its high thermal conductivity of 220 W/mK. This value is nearly equal to that of beryllia. The electric insulation ability and the mechanical strength of aluminum nitride are equivalent to those of alumina.
A sintered body of aluminum nitride (AlN), having a high thermal conductivity and an excellent mechanical strength, is being watched as a material which is usable as an insulating substrate for a semiconductor integrated circuit device (IC) and as a substrate for forming an electric circuit, such as a power module. However, since such an aluminum nitride sintered body is inferior in its wetting property relative to a metal, so that a sufficient adhesive strength cannot be obtained in laminating a metal layer on the surface of an aluminum nitride substrate for a semiconductor integrated circuit such as a power transistor. Although various attempts have been made to metallize the surfaces of such AlN sintered compacts, no satisfactory method has been proposed so far.
A technique relating to such an aluminum nitride sintered body is disclosed in U.S. Pat. No. 4,547,471, for example. Further, Japanese Patent Publication No. 121175/1984, for example, discloses a method for metallizing the surface of an aluminum nitride sintered body obtained through said technique.
This prior art discloses generally to use a molybdenum paste and a copper paste, although compositions thereof are not concretely defined. Japanese Patent Publication No. 132580/1986 discloses a method for metallizing a nitride ceramic body which has formed on its surface a metallized layer by adding Mo, W, Mn, etc. to MgO, AlN, Y.sub.2 O.sub.3 or SiOn. Japanese Patent Publication No.: 105972/1987 discloses a method for forming a cracked oxide layer on the surface of an aluminum nitride sintered body and thereafter coating a vitreous adhesive agent on the surface of the oxide layer.
Further, Japanese Patent Publication No. 75208/1975 discloses a method of metallizing the surface of an aluminum nitride sintered body by oxidizing the surface of the aluminum nitride sintered body and sintering a metal such as Mo, W, Mn or Ti on the surface thereof. Japanese Patent Publication No. 102310/1978 discloses a heat-conductive substrate comprising a sintered substrate of aluminum nitride and a metal layer of Mo, W, Mo-W system or an Mo-Mn system formed as a metal oxide layer such as an oxide layer containing SiO.sub.2, Al.sub.2 O.sub.3, MgO, CaO or Fe.sub.2 O.sub.3, for example, formed on a prescribed surface of the substrate.
On the other hand, the so-called Telefunken method of coating a paste of tungsten or tungsten-manganese (or molybdenum or molybdenum-manganese) on the surface of a sintered body and firing the same in wet hydrogen or a wet H.sub.2 --H.sub.2 gas mixture at a temperature of 1300.degree. to 1700.degree. C. is well known as a technique for metallizing a sintered body of aluminum oxide (Al.sub.2 O.sub.3).
The Telefunken method is characterized in that the Al.sub.2 O.sub.3 sintered body is fired in a wet atmosphere at a temperature sufficient for softening the Al.sub.2 O.sub.3 sintered compact to its glassy plane.
The surface of W and/or Mn is oxidized by such a firing, to accelerate the sintering of the paste of W or W-Mn. Oxides of such materials are dissolved in the glassy phase of the sintered body to improve the flowability of the glassy phase, whereby the glassy phase is transferred to a porous metallized layer of W or W-Mn. Further, the oxides generated by the firing, particularly MnO reacts with Al.sub.2 O.sub.3 and SiO.sub.2 contained in the sintered body, to form MnO.multidot.Al.sub.2 O.sub.3 and MnO.multidot.SiO.sub.2. Similarly, W is partially oxidized to generate tungsten oxide, which strongly reacts with alumina. Thus, the metallized layer of W or W-Mn (or Mo or Mo-Mn) strongly adheres to the Al.sub.2 O.sub.3 sintered body through mechanical and chemical bonding, with an adhesive strength of about 4 to 7 kg/mm.sup.2.
Thus, it may be assumed that an AlN sintered body can be metallized through the aforementioned Telefunken method. According to that method, however, a metallized layer of W or W-Mn formed on the surface of an AlN sintered body has merely a low adhesive strength and the airtightness is extremely inferior. Possible causes therefor are as follows:
(a) Since firing is performed in a wet atmosphere according to the Telefunken method, the surface of the AlN sintered body is corroded by steam or decomposed to form a fragile Al.sub.2 O.sub.3 layer. PA1 (b) Dissimilarly to the Al.sub.2 O.sub.3 sintered body, the AlN sintered body is not provided with any glassy phase, etc. which is softened at a low temperature of about 1000.degree. to 1500.degree. C. PA1 (c) AlN is inferior in fits ability to react with W, Mn and oxides thereof.
When the metallized layer formed on the surface of an AlN sintered body, which is applied to a substrate for a semiconductor integrated circuit device, has a small adhesive strength, the metallized layer is easily peeled off by a heat applied in the manufacturing steps. Further, insufficient airtightness of the metallized layer leads to an inferior strength and to an insufficient sealing property. Finally, the thermal resistance of the metallized layer is higher, i.e. less desirable, when the layer does not adhere well to the substrate.
In order to apply the Telefunken method to the metallization of an AlN sintered body, the present inventors have attempted to add glass and other presumably suitable materials such as Y.sub.2 O.sub.3 or CaO to a paste of W or W--Mn for firing the same in an inert atmosphere. However, it has been impossible to solve the aforementioned problem relating to adhesive strength and airtightness. When employing glass, the metallized layer did not strongly adhere to the sintered body due to an inferior wetting property of the remaining W or Mn and AlN with glass. When employing Y.sub.2 O.sub.3 or CaO, a reaction layer was slightly formed at a firing temperature of at least about 1600.degree. C., but the metallized layer thus obtained was extremely porous and its adhesive strength was low due to the high melting point and no liquid phase was formed in the metallized layer.
U.S. Pat. No. 4,835,039 (Barringer et al.) discloses a tungsten paste for co-sintering with pure alumina and a method for producing the same. The method according to Barringer et al. involves mixing tungsten powder with an alkaline earth aluminosilicate glass powder having 10 to 38 wt. % alkaline earth, 10 to 52 wt. % alumina and 10 to 70 wt. % silica. The mixture is applied to an alumina tape and fired at between 1450.degree. C. and 1550.degree. C. in a wet atmosphere of dissociated ammonia and nitrogen (50% H.sub.2 and 50% N.sub.2) and water vapor.
U.S. Pat. No. 4,493,789 (Ueyama et al.) discloses an electroconductive paste and a method for producing metallized ceramics using the same. The method according to Ueyama et al. involves mixing 100 parts by weight of a high melting metal powder, such as tungsten powder, with 0.1 to 3 parts by weight of an additive mixture including MgO, CaO, SiO.sub.2, Al.sub.2 O.sub.3, for example. The mixture is coated on an alumina substrate and is then fired at a temperature of 1400.degree. C. to 1700.degree. C. in an atmosphere containing water and a mixture of gases, such as N.sub.2 and H.sub.2 gases.
The metallized layers produced according to Barringer et al. and Ueyama et al. do not achieve the low thermal resistivity of the present invention, as will be discussed below. Furthermore, as described above, the present inventors have determined that the wet firing methods are not compatible with aluminum nitride sintered bodies for achieving good metallization results thereon.